VARA HF , TRI MODE and the SDR
The use of SDR and the VARA HF modem is limited to , SDR one side of the connection only
ie Conventional <> SDR , or SDR <> Conventional
Where as VARA FM and VARA Sat , support SDR<>SDR
As V modem deploys dynamic timing , adjusting for symmetrical & asymmetrical
link delays , this at first sight, looked a little 'odd
considering, the FM and Sat versions are 100% compatible
It transpires that VARA HF latency cannot be extended , due to the
requirement to be compatible with TRI Mode band scanning limitation of 4 seconds
in which, the delay from only, one SDR may be accommodated ..this may not
be fully recognised and could present un-expected issues ..
VHF 'FM' 1.2/9.6 Unaffected
HF gateways deploying SDR may fail to connect with SDR clients
conventional tx/rx may be required to provided conventional & sdr connections
VARA-HF-Chat SDR<>SDR may not function *
*NB- VARA -SAT will function with Chat SDR<>SDR
over HF link -
Unless the SDR processing times are reduced and/or the 4 second Tri-mode scan
issue is addressed , the situation remains precarious , with legacy issues influencing
HF gateway configuration .. extending the V-HF latency [accommodation] is at odds with
the Tri-Mode scan cycle ... catch 22
Are Winlink aware of this issue? And do they plan to address it?
The Trimode Dwell time of 4 seconds , is the 'only' limiting factor , preventing the deployment of SDR in a gateway
which is perhaps a legacy issue ..
Unlike 'fixed'/'static' timed systems / protocols , Processing delays and switching times are not critical, the Vara modem deploys 'Dynamic' timing , additional delays [symmetrical & asymmetrical]
are automatically compensated for . eg repeater delays for VHF/FM and the long path delay , for the AO-100 Sat link version [ which also can be used for HF etc ] , leaving the HF version, while compatible with the 4 second dwell requirement ,unable to provided otherwise, sdr<>sdr connectivity
The limiting factor is solely the 4 seconds delay , [as to why '4' seconds is required
is not clear ?] .
Increasing the Dwell time by 500mS , to 4.5 seconds , would allow deployment of the Vara HF
modem , inside the Trimode system and provide both SDR<>SDR and Hybrid connectivity
both inside and out side the winlink system.
SDR latency is a little longer than conventional , But , as is , its not possible for , say,
a two way connection , using , Flex SDR<> Flex SDR and the Chat module
probably a case of unexpected consequence ..
Clarification from Jose ,
3 May 2021, 18:48:23 (yesterday)
to VARA HF Modem
Yes, with Dwell Trimode 4.5 seconds it would be possible to do a simple change to VARA HF to connect two SDRs: SDR<->SDR. Although the latency in this case would be about 1 second due to both VAC.
But Winlink demanded 4 seconds maximum Dwell, so only is possible SDR<->Conventional or Conventional <->SDR.
In VARA FM or VARA SAT is possible SDR<->SDR because there is no DWELL time requirements.